In the prior art, a partially added cyclic organo-hydrogensiloxane is prepared by partially adding a compound having an aliphatic unsaturated hydrocarbon group to a cyclic organohydrogensiloxane in the presence of a catalyst such as a platinum compound. In this process, the addition reaction product is isolated by distillation, often in low yields.
More particularly, a partially added cyclic organo-hydrogensiloxane is prepared by partially adding a compound having an aliphatic unsaturated hydrocarbon group of the general formula (4) to a cyclic organohydrogensiloxane of the general formula (3) in the presence of a platinum compound catalyst according to the following reaction scheme. ##STR1##
Herein, R is a monovalent hydrocarbon group, n is 3 to 5, and m is 1 to 2. This process, however, suffers from the phenomenon that when the reaction mixture is heated for isolating the reaction product by distillation, ring-opening reaction occurs on both the reactant and the product whereby the isolation yield of the end product becomes very low.
The ring-opening reaction upon heating takes place through the following mechanism. The platinum compound added as the addition reaction catalyst catalyzes ring-opening reaction with a minor amount of water in the system to form silanol compounds, which trigger ring opening of the cyclic organohydrogensiloxane (reactant) and the partial addition compound (reaction product).
To restrain the ring-opening reaction, JP-B 51-33540 proposes to control the catalytic activity of a platinum compound by adding a nitrogen, phosphorus or sulfur-containing compound which becomes a catalyst poison.
The inventor found that even after the catalytic activity is lost, some molecules can exceed the activation energy barrier during heating. As isolation by heat distillation is continued for a long time, ring-opening reaction slowly takes place. Because of this phenomenon, the method is unsatisfactory.
Therefore, for the preparation of a partially added cyclic organohydrogensiloxane, it has been desired to have a technique capable of improving the yield of distillative isolation.